Alarm production over broadcasting channel by using long duration dissonant tones discordant with musical scale to prevent false actuation



July 2, 1968 G. w. FYLER ETAL 3.391,340

ALARM PRODUCTION OVER BROADCASTING CHANNEL BY USING LONG DURATION DISSONANT TONES DISCORDANT WITH MUSICAL SCALE TO PREVENT FALSE ACTUATION Filed May 19. 1964 United States Patent O 3,391,340 ALARM PRODUCTION OVER BROADCASTING CHANNEL BY USING LONG DURATION DIS- SONANT TONES DISCORDANT WITH MUSI- CAL SCALE TO PREVENT FALSE ACTUATION George W. Fyler, Lombard, and Karl E. Hassel, Elmhurst, Ill., assignors to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed May 19, 1964, Ser. No. 368,510 3 Claims. (Cl. S25- 64) ABSTRACT OF THE DISCLOSURE An alarm system for a radio receiver wherein an alarm is actuated upon the concurrent reception of two mutually dissonant tones for a predetermined minimum time interval. By selecting tone frequencies different from those of the standard musical scale, optimum security against false actuation is achieved and by making the tones dissonant, an audible alarm is achieved by reproducing the received tone signals through the receiver speaker. This system, because of its economical circuitry and security against false actuation, is especially well suited for a national alerting system wherein the receiver may be maintained tuned to a standard broadcast station with its speaker muted.

The present invention relates to alarm or alerting systems and particularly to a radio alarm which combines both surety of actuation and security against false actuation.

In a radio alarm system, one of the primary advantages is that the existing radio network may be used to give virtually instantaneous notification of an alarm situation to nearly the entire population. Additionally, the alert could be followed by verbal instruction to the listener. Since virtu-ally every home in America has at least one AM or FM radio receiver, it W-ould be possible to alert virtually the entire population of an area or of the country in the event of an emergency. Should a danger arise, such as an approaching hurricane, impending flood or, even more importantly, a sneak nuclear attack, alerting the public and giving it instructions could result in the saving of thousands or millions of lives.

A useful form of such a system envisions radio receivers which are always maintained in a standby or muted condition when not normally receiving programs but which, in the event of an emergency, are lactivated by a signal from a radio broadcasting station. In this case, however, it would be extremely undesirable for the radio to undergo false actuation. It therefore is necessary to provide considerable security against false actuation, both to avoid inconvenience to the user, and to serve as a stimulus to encourage maintenance of the alert-equipped radio in a standby condition. vIt is all too likely that the public would soon eschew the maintenance of an alert radio in standby conditions should this radio create any inconvenience. It might not require more than one occasion of the average user being awakened by a false actuation to persuade him to abandon using the alerting feature.

For similar reasons, it is desirable to provide ordinary home radios with the `additional function of serving as an alert radio and to minimize the additional costs involved in converting present radios or in incorporating the alert feature into future radios. In connection with the installation of the activating signal apparatus in the broadcasting stations, it is likewise desirable that the additional equipment expense be kept to a minimum.

A secure but economical alert system also finds utility in a number of other situations where notification and 3,391,340 Patented July 2, 1968 ICC subsequent communication are desirable. For example, many small airports or landing strips in the United States maintain no otiicial radi-o station. There often is, however, a Unicom, a private radio station oiiicially designated as an aeronautical advisory station. Such Unicom stations are commonly maintained by the operator of the airport service station as an aid for private pilots who may become customers. Through such stations, vital information as to wind, weather and runway conditions is obtainable and, in addition, arrangements for other services such as rental cars and hotel reservations are often available. A colnmon practice among the operators of such stations is to maintain the radio receiver at low volume so as not to interfere with otiice operations. Consequently, communications from incoming airplanes may be missed when the Unicom operator is engaged in activities away from the receiver. It would therefore be an advantage if such radios were to have their speakers activated only upon the reception of an intended signal. Under such conditions, the standby audio output level could be maintained at a higher value so as to increase the distance of its effectiveness. Further, it would facilitate the utility of such radio communication service if the system also called attention to the reception of a proper incoming signal by affording a special warning tone or sound.

It is a general object of the invention to provide a reliable alarm system having a maximum of security against false actuation.

It is another object of the invention to provide a reliable alerting system for activating a receiver output only in response to a preselected signal.

It is still another object of the invention to provide a radio alarm that is both reliable in actuation and secure against false actuation.

It is a further object of the invention to provide such a radio alarm or alerting device that may be easily incorporated into present or future radio receivers at a minimum of expense.

In accordance with the invention, an alarm with a high degree of security against false actuation is achieved by the transmission of a plurality of selected audiofrequency signals. These signals are selected to have different frequencies discordant with the frequencies of the notes in conventional musical scales. The transmission is received and at least two such selected audiofrequency signals are sensed. In response to the receipt and detection of the two signals, an alarm or lother indicating device is activated.

The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawing, in the tigures of which like numerals identify like elements, and in which:

FIGURE 1 is a schematic diagram in block form of a radio broadcast stati-on incorporating features of the invention;

FIGURE 2 is a schematic diagram of a radio receiver incorporating the invention; and

FIGURE 3 is a schematic diagram of an alternate ernbodiment of part of the receiver in FIGURE 2.

Transmitter FIGURE 1 depicts a radio broadcasting transmitter. It includes a radio-frequency oscillator 5 for generating a carrier signal and conventionally may also include a number of amplifier stages for amplifying the carrier signal. The carrier signal generated by oscillator 5 is fed to a nal amplifier 5a. The carrier signal in amplifier 5a is modulated with a composite audio signal delivered to amplifier 5a by a modulator 6. Modulator 6 is driven with broadcast program signals from audio program channel 7. The program channel conventionally provides live or recorded music and speech. The composite output signal from amplifier Sa includes the carrier signal amplitude modulated in response to the signal from program channel 7. This modulated carrier is fed to antenna 8.

In accordance with the invention, modulator 6 is fed with at least two additional signals developed in oscillator 9a and oscillator 9b, each of which develops a separate signal of a selected fundamental frequency. The selection of these signals will be described later in more detail. For the present, it will suffice to note that oscillator 9a and oscillator 9b preferably develop square-Wave audio signals having fundamental frequencies which are discordant and produce a dissonant, raucous sound when audibly reproduced.

When operating conventionally, audio programming from program channel 7 is fed directly to modulator 6 to impress the program upon the broadcast signal. When it is desired that an alarm should be sent, controller 9c activates oscillators 9a and 9b which feed the two signals generated to modulator 6 which in turn modulates the main carrier with these signals. In a typical arrangement, controller 9c operates oscillators 9a and 9b concurrently for approximately l5 seconds each. During this time, controller 9c also may be operated to disable program channel 7. Following the transmission of the signals from oscillators 9a, 9b, audio information from program channel 7 is fed to modulator 6 to enable broadcast in a conventional manner of spoken comment or warning.

A frequency-modulated radio transmitter or a television transmitter could just as well be used for alarm system purposes. In the case of an FM system, the signal from modulator 6 typically is fed to oscillator 5 to modulate the frequency of the carrier signal in response either to the audio programming from audio channel 7 or, upon direction of controller 9c, to the signals developed by oscillators 9a and 9b.

Receiver FIGURE 2 illustrates a radio embodying a preferred embodiment of the invention. The radio itself may be either a conventional AM or FM home receiver and may obtain power from a conventional battery or from the AC lines. It is equipped with an antenna y10 and has entirely conventional tuning, demodulating and amplifying circuits which collectively are designated as receiver 11. Receiver 11 has its audio output terminals connected to a conventional audio transformer 13. In most ordinary radio receivers, the voltage induced across the output terminals of transformer 13, between terminal 14 and ground, is directly applied to the voice coil of a reproducer such as speaker 15 so as to develop the sound output of the receiver. In this case, a manually-operable switch 16 in one position selectively removes return of the voice coil of speaker 15 from ground. In the other position, switch 16 permits conventional operation of receiver 11 together with speaker y15.

A reed-relay coil 17 is coupled to the output terminals of transformer 13. Coil 17, in a preferred physical embodiment, surrounds reed set 18 which includes a plurality of metal reeds, in this case two reeds 18a, 18b. The reeds are constructed so as to vibrate in response to signals induced in coil 17; they are resonant (vibrate with maximum amplitude of vibration) at two carefully selected frequencies, e.g., 520 c.p.s. and 570 c.p.s.

Across coil 17 is a diode pair 17a, the diodes being connected in parallel with opposing polarities. While not strictly necessary to the operation of the circuit, they have been found to increase its security against false actuation. Should the amplitude of the voltage across coil 17 exceed a small value in either direction (determined by the threshold of the diodes), either one or the other will conduct to pass any excess current. Since the diodes function to shunt current around coil 17 when the voltage exceeds a selected minimum, the reed response is one of essentially constant bandwidth. Consequently, regardless of audio frequency, the strength of the induced vibrations of reeds 18a and 18b is kept to a safe minimum to prevent reed breakage and excess wear. A capacitor 19 coupling coil 17 to a contact on switch 16 serves to block direct current and to form a series resonant circuit with coil 17 in the general vicinity of the alert signal frequencies.

Coupled to a normally open Contact associated with reed 18a is a negative D C. power source -B; it is connected to the contact through two series-connected resistors 20a and 20b. Source -B is preferably the power supply of receiver 11, but could, of course, be a separate battery. An alternating current bypass capacitor 21, connected from the junction between resistor 20a and resistor 2Gb to reed 18b, serves to reduce transients and radio frequency interference from the reed contacts. A normally open contact associated with reed 18b is returned to the B-lterminal of the power supply, in this case ground, through a resistor 23b and the normally closed contacts of a relay 24. A capacitor 23a is connected across resistor 23b and together with resistor 20 (resistors 20a and 2011 considered together) constitutes a timing or delay circuit to be explained more fully hereinafter. For convenience, capacitor 23a and resistor 23b are designated as delay circuit 23. A capacitor 25 is connected from the junction between circuit 23 and the relay 24 terminal to ground and this junction is returned to B- through a resistor 26.

The side of capacitor 23a connected to the contact associated with reed 1817 is connected to ground through a manually-operable switch blade 27 ganged to the blade of switch 16 so as to ground the capacitor when switch 16 connects the coil of speaker 15 to ground. A pnp transistor 28 has its base connected to the junction 22 between capacitor 23a and the contact associated with reed i18b, its collector connected directly to source -B, and its emitter connected to the base of a second pnp transistor 29. The base of transistor 29 and the emitter of transistor 28 are conected through a load resistor 30 to ground. The collector of transistor 29 is connected through the operating coil 31 of relay 24 to source B-, while its emitter is returned to ground through a resistor 32. and switch 16 when the switch is in its condition to remove the speaker coil return from ground; when switch 16 is in its alternate position, the emitter circuit effectively is opened. For convenience of explanation, the common terminal between the reed 18b contact, the terminal on switch 27, delay circuit 23 and the base of transistor 28 has been designated by the number 22.

FIGURE 3 depicts an alternate embodiment of reed set 18, incorporating an additional reed 18e and a selective switch 33. The dual reed set of the FIGURE l circuit permits response only to one signal pair. The advantage of the reed arrangement in FIGURE 3 is that it allows the number of possible alerts or alarms in response only to the receipt of a pair of signals to be extended from one to three. For example, one reed pair, 18a and 18b, which always is connected in series no matter Which setting of switch 33 is selected, could be assigned to respond at the time of national emergency alerts (setting 2 of switch 33); whenever signals corresponding to the frequencies assigned to reeds 18a and 18b are received, a periodic series connection is established from resistor 20 to delay circuit 23. For setting 1 of switch 33, reed pair 13b and 18C could be assigned for local emergency alerting and similarly would complete the aforementioned series connection, the response of reed 18C being to a third discordant signal from the transmitter (e.g., 640 c.p.s.). And the third pair, 18C and 18a (setting 3 of switch 33), could be assigned to respond at the time of ordinary news bulletins. Alternatively, one of the three alert frequency combinations could be assigned to announcements Ifor doctors, volunteer firemen, auxiliary policemen, or the like.

Operation The circuit of FIGURE 2 is shown in the standby condition, awaiting an alarm. In this condition, it is tuned to an ordinary broadcast station and is receiving and detecting the normal broadcast signals transmitted by the station. However, speaker 15 is silent since switch 16 is open. A signal corresponding to the audio out-put is developed across coil 17.

Reeds 18a, 18b are constructed in this instance to vibrate resonantly at 520 c.p.s. (cycles per second) and 570 c.p.s. They preferably have a Q, or quality factor as a narrow bandwidth device, of 100. That is, for the tuned frequency of 520 c.p.s. the bandwidth of the reed is 5.20 or 1% of 520 c.p.s.; for 570 c.p.s., the lbandwidth is 5.7

c.p.s.

When the broadcast transmitter signal includes the two alarm frequencies, reeds 18a and 18b vibrate against their associated contacts. Each reed is closed with its contact during approximately 20% of the time when the reed is vibrating. Since the reeds are in series with each other and respond to different signals, the circuit through reed set 18 is closed approximately 4% of the ttime. During this short period, current flows through the reeds and circuit 23. This current develops a charge on capacitor 23a. At the same time, resistor 23b tends to equalize the charge on capacitor 23a. However, as a current of suicient magnitude persists a net negative voltage with respect to ground is developed at terminal 22. The negative potential developed at terminal 22 drives transistor 2S, causing current to flow from ground through resistor 30 to develop a potential which in turn drives transistor 29. This in turn induces current from ground through switch 16, resistor 29 Iand relay operating coil 31.

The different resistor and capacitor values in the reed relay circuit are selected so that the charge on capacitor 23b reaches a level sufficient to turn on transistor 28 only after a selected time period. Thus, resistor 20, resistor 23b and capacitor 23a form a delay or timing circuit governing the operation of relay 24. The values of these circuit elements are preferably chosen to provide a delay of approximately 5 seconds after receipt of the reed-resonating signal before actuation of relay 24. Upon such actuation, the audio output from the receiver is thereupon reproduced by speaker 15. Also when relay 24 is actuated, current through resistor 26 from source B- causes anegative potential to build up Iacross capacitor 25 and the potential at terminal 22 is in turn increased more negatively. Consequently, the two transistor stages are driven into still greater condition to insure positive closure of the armature of relay 24 against its contact connected to the voice coil of speaker 15 and to hold relay 24 in its actuated condition following termination of the receipt of the two alert signals. The circuit remains in its on state until such time as switch 16-27 is thrown to its vposition in which the speaker voice coil and terminal 22 are returned directly to ground. The value of capacitor 25 is comparatively large so that momentary premature opening of the lower contacts of relay 31, as might be caused by vibration of the unit, does not permit any significant charge build-up across capacitor 25 which otherwise might cause false actuation of the alert system.

The speaker output includes the two alert signals sent by the broadcast station to activate the speaker and may include other notes and/or harmonics, The effect of such notes and noise is to produce a harsh discordant sound which serves as the notifying alarm. The two frequencies chosen are such that they produce a beat frequency of approximately 50 c.p.s. and therefore are dissonant.

As envisioned, the broadcast station transmits the two altering frequencies for a period of approximately 15 seconds. Upon reception of the signals by the receiver 11 for a period of approximately 5 seconds, determined by the timing circuit composed of resistor 20, resistor 23b and capacitor 23a speaker 15 is activated and for a l0 second interval produces the dissonant sound broadcast including the two alerting signals.

Following the broadcast of alerting signals, the speaker remains in condition to reproduce any additional information sent out by the station. This could be information as to the type of danger and instruction to the hearing. To allow only a comparatively short period of actuation of speaker 15 following reception of the activating signal, a switch 25a is connected across capacitor 25 to permit selective shorting of the latter. With switch 25a closed, relay 24 remains actuated following cessation of the alert signals only for the period of time determined by the discharge time-constant associated with capacitor 23a.

To use the radio of FIGURE 1 as an ordinary radio or to prepare it for return to the standby condition, the user throws ganged switches 16 and 27 to the position directly returning the voice coil of speaker 15 to ground. Operation of the speaker is then independent of relay 24 and coils 17 and 31 have their ground returns opened, as a result of which relay 24 opens. The charge on capacitor 25 is thereupon dissipated through the lower contact on relay 24. At the same time, switch 27 closes to discharge capacitor 23a directly to ground. All elements are then conditioned so that, upon again throwing switch 16, 27 to the standby position, the unit is ready to accept and act in response to receipt of the alerting signals.

Frequency selection As indicated above, a special feature of the invention lies in the selection of the alert signal frequencies in order to give special security against false actuation as well as surety of actuation when these signals are received. The frequencies are selected from the normally reproduced audio frequency range and do not include the frequencies of any notes of the standard or conventional musical scale nor any harmonic of the power frequency, normally 60 c.p.s., that are within the response bandwidth of the vibration reeds. Preferably, the signal frequencies should be within the range of the audio reproduction of even poor-quality radio receivers and they should likewise avoid the standard musical scale frequencies by as great a margin as possible. In generally is undesirable to place one or both of the signals below or above the audio range, since this might require redesigning the receiving circuitry in many radios to facilitate reception, detection and sensing of the non-audio signal.

It is desirable for saving in equipment cost and for eiciency of operation to use the alerting signals directly as the sound output of the speaker, since this requires no additional alarm indicating devices. When desired, however, a separate alarm indicator, such as a buzzer or light, may be actuated by contacts on relay 24.

The signal frequencies used in the instant embodiment are midway between the frequencies of conventional musical notes and are preferred because they lie Within the range of psychological loud sounds, that is, they sound relatively loud to the human ear. Psychological tests have shown that among sounds of equal physical amplitude those of approximately the preferred frequencies sound loudest to the average human ear. The frequencies are also preferably chosen to be dissonant, that is, to create a displeasing effect when played together. The noise produced as an alarm therefore is raucous and more startling. The dissonant sounds produced because of the particular frequency selection serve better as an alarm than more pleasing sounds. This psychological fact has heretofore been utilized in the construction of sirens, horns and whistles. It is an added advantage, therefore, that the activating signals are employed directly as the alarm, since no additional alarm equipment is required; in other words, speaker 1S also serves as the alarm indicating device.

Further, the use of discordant notes decreases significantly the likelihood of response to notes played in musical programs. The range of normal human hearing is approximately from 16 c.p.s. to 16,000 c.p.s. Within that range the average ear can distinguish about 1,400 discrete frequencies. But in both Western and Oriental music the number of musical tones has been quantitized at a much smaller number. For example, in the scale of equal temperament this number is 120. This quantitization is probably the result of the fact that most musical instruments are resonant physical systems which by their natures can be resonant only at a certain number of discrete frequencies. By tradition and convention, the musical tones have been assigned only certain frequencies while others have been neglected. Using conventional musical notation and instruments tuned to the international standard, a composer cannot successfully write a number using non-conventional notes. Similarly, a properly tuned piano does not produce a note of 512 c.p.s. or 5 68 c.p.s.; it would have to be out of tune by nearly 50% before the selected alarm frequency would be produced. Such unconventional music as certain modern jazz still use conventional note scales.

While the musical scale is quantitized at discrete frequencies, the choice of a standard frequency is of late arrival. Until 1939, A was commonly set at 435 c.p.s. for many orchestras and for scientific purposes, while it was set at 461.6 for other uses and some orchestras. Over the past centuries, various standard pitches have been adopted by different musical groups. The inconveniences arising from this situation were overcome as the result of action by an international convention held in 1939; for example, it established 440 c.p.s. as the standard for the note A. Today, this standard is virtually in universal use and the system of the present invention is therefore preferably employed with that in mind. It is to be noted, however, that the preferred frequencies illusstrated above do not correspond to notes in the former conventional musical scales.

While it may be likely that a tone having a frequency the same as that of one or the other alert signals might be generated by an instrument such as a trombone in passing from one note to the next, the likelihood of two such signals being generated at the same time during `a musical rendition is small. Further, the likelihood that two such signals, both dissonant with each other, would be generated for 4the delay period, e.g., 5 seconds, is so extremely small that nearly perfect security against false actuation is afforded.

While the choice of the musical scale notes was one of convention, the phenomenon of consonance and disconsonance is founded in the psychological effect of the sounding of two notes together or sequentially. The impression of dissonance occurs when the notes sounded together produce a beat frequency between and 60 c.p.s. By choosing the two alarm frequencies so that, in addition to the other considerations, they create a beat frequency so as to sound dissonant, the probability of a sustained sounding of the two notes in a musical composition is still further minimized, while the psychological effect of the alerting function of the audible alarm is increased.

As mentioned, the response of the above-described circuit may be used to activate other devices in addition to or alternatively to speaker 1S. One useful approach is to alter the tuning of receiver 11 to a special broadcast station in response to receipt of the alarm signals. The receiver may then be tuned at any time to any one of a number of broadcast stations equipped to sound the alert and upon its receipt be automatically switched to a station specially designated to give verbal news or instructions. Alternatively, receiver 11 may remain tuned to the one specially designated station as well as to a different userselected station with the provision of means for disabling response to the special station except upon receipt of the alarm signals.

The particular apparatus illustrated and explained with respect to FIGURE 2 has been found to operate with great effectiveness. At the same time, it is capable of being manufactured economically and requires but small physical space. However, the invention contemplates the use of other kinds of receiving apparatus in conjunction with the receipt of the particularly selected alerting signals. For example, in one arrangement the alerting signals are detected and selected by individual filters assigned respectively to the two or more different alerting signals. The signal delivered through each iilter preferably is integrated to afford time delay and a conventional coincidence gate is employed to operate only upon the concurrent reception of both signals for the period of time determined by the integration circuit, a predetermined energy accumulation in the latter being required to overcome the threshold value of the coincidence circuit. Alternatively, one of the received signals is employed in known manner to arm a relay or gating circuit with the other signal being utilized to actuate that circuit. With this arrangement, concurrent receipt of the two signals is not necessary, although integration preferably is included to prohibit ultimate alarm indication unless each of the two signals persists for a selected minimum time period. In still another alternative, the signals are transmitted as a series of pulses and a counter is used in the alarm circuit to respond only after receipt of a selected minimum number of such pulses. The construction and manner of operation of all of these different alternative arrangements is well understood by the person skilled in this art, and, therefore, does not require any more detailed explanation. In all cases according to the invention, it is the particular frequency selection which yields a primary element of security against false alarm; the aforementioned protection afforded by integration, coincidence, counting and the like, and any one or more may be employed in a given unit, contributes additionally to the amount of security in the system. Moreover, it is contemplated to utilize the alert signal circuitry in conjunction with a receiver sensitive essentially only to those signals. This approach, of course, does not have the advantage of a direct tie-in with normal broadcast reception.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that other changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. In connection with the operation of a radio broadcasting system based on transmission of a carrier signal modulated with a program signal which comprises music played with notes in a standard musical scale based on A=440 cycles per second, the method of producing an yalarm which comprises:

concurrently modulating said carrier signal with a plurality of mutually dissonant audiofrequency tone signals each `of a duration of the order of 1S seconds .and each of a frequency discordant with said notes of said standard musical scale;

receiving and detecting said modulated carrier signal to derive said program signal and said tone signals therefrom;

developing a control effect in response to the concurrent persistance of the derived tone signals for a period of the order of 5 seconds;

and utilizing said control effect to effectuate audible reproduction of said dissonant tone signals for the balance of their duration and audible reproduction of said program signal thereafter.

2. A radio broadcasting system including a transmitter for broadcasting a carrier signal modulated with a program signal which comprises music played with notes in -a standard musical scale based on A=440 cycles per second, and a receiver for receiving and detecting said program signal, the improvement which comprises:

means at said transmitter for concurrently modulating said carrier signal with a plurality of mutually dissonant audiofrequency tone signals each of a duration of the order of 15 seconds and each of a frequency discordant with said notes of said standard musical scale; a loudspeaker at said receiver; means at said receiver for eifectively disabling said loudspeaker; means at said receiver for receiving said modulated carrier signal and detecting said program signal and said tone signals; means coupled to said receiving and detecting means for developing a control effect in response to the concurrent persistence of said audiofrequency tone signals for a period of the order of 5 seconds; and means coupled to said last-mentioned means and responsive to said control effect for applying said detected audiofrequency tone signals to said loudspeaker for the balance of their duration to create a dissonant audible alarm, and for conditioning said loudspeaker to reproduce said program signal thereafter.

3. The system of claim 2, in which said means for developing a control effect comprises a plurality of resonant reeds respectively tuned to the frequencies of said tone signals and each having a Q of at least substantially 100', in conjunction with a time delay network for developing said control effect in response to continuous concurrent vibration of said reeds for a period of the order of 5 seconds.

References Cited UNITED STATES PATENTS 2,579,470 12/1951 Brown 325-466 2,617,923 11/1952 Rekart 325-64 X 3,043,916 7/1962 Sneath 325--64 X 3,123,675 3/1964 Boone et al. 179-87 3,264,634 8/1966 Voight 340-171 X ROBERT L. GRIFFIN, Primary Examinez'.

JOHN W. CALDWELL, Examiner.

B. V. SAFOUREK, Assistant Examiner. 

